1. Field of the Invention
The invention relates to a method and system for treating contaminated media. In particular, the invention relates to a method and system employing an electrode structure that is capable of controlling a treatment method in contaminated media.
2. Description of the Related Art
Halogenated hydrocarbons, such as chlorinated hydrocarbons, are also known as chlorinated solvents (hereinafter collectively referred to as "chlorinated solvents"). Halogenated hydrocarbons have low flammability and are fairly stable, both chemically and biologically. They are commonly used in industry as chemical carriers and solvents, paint removers, and cleaners. The cleaning applications typically include metal degreasing, circuit board cleaning, metal parts cleaning, and dry cleaning. Chlorinated solvents are also used as intermediates in chemical manufacturing and as carrier solvents for pesticides and herbicides.
Chlorinated solvents are stable compounds, are relatively toxic at low levels, and many chlorinated solvents have been classified as suspected or confirmed carcinogens. Chlorinated solvents are prevalent contaminants in groundwater and soil because of their widespread use and long-term stability. Groundwaters and soils have become contaminated by chlorinated solvents from various sources. These sources include, but are not limited to, disposal facilities, chemical spills, and leaking underground storage tanks. Chlorinated solvents also may be released to the environment through the use, loss, or disposal of a neat liquid, and alternatively through the use or disposal of wash and rinse waters containing residual solvents.
Movement and dispersion of chlorinated solvents in the subsurface soils and groundwaters vary depending on whether the solvents are released as a neat liquid or in a dissolved form. If released in a dissolved form, chlorinated solvent migration is governed largely by hydro-geological conditions and processes. The presence of solubilizing agents, such as soaps from wash waters, counteracts natural soil sorption-retardation mechanisms for chlorinated solvents, and enhances migration of the chlorinated solvents.
If chlorinated solvent is released as a neat liquid, the chlorinated solvent migrates through soil under the force of gravity. A portion of the chlorinated solvent is typically retained in soil pores. If sufficient chlorinated solvent is present in the soil, the soil pores become saturated. Additional chlorinated solvent continues to migrate in the soil until it encounters a physical barrier or a water table. If the chlorinated solvent encounters a water table, the chlorinated solvent disperses until it encounters, accumulates, and overcomes the water table's capillary forces. At this point, the chlorinated solvent, which has a greater density than water, penetrates the water table's surface. The chlorinated solvent migrates under the force of gravity until its amount has been diminished through sorption, or until the chlorinated solvent encounters an aquitard.
In recent years, soil and groundwater contamination by chlorinated solvents has become an environmental problem. Chlorinated ethylenes, such as trichloroethylene (TCE), tetrachloroethylene (commonly known as perchloroethylene (PCE)), and chlorinated ethanes, such as 1,1,1-trichloroethane (TCA), which have been used as degreasing agents in a variety of industrial applications, pose environmental problems. Even though chlorinated degreasing agent use was curtailed in 1976, improper storage and uncontrolled disposal practices have resulted in contamination. Due to the high water solubility of chlorinated solvents, for example about 1100 mg/L TCE at 25.degree. C., chlorinated solvents are highly mobile in soils and aquifers, and should be removed before dispersing too far. Therefore, a treatment to remove chlorinated solvents from contaminated soil and groundwater is needed.
A pump-and-treat method is a proposed treatment method removing contaminants from contaminated groundwater. The treatment usually involves withdrawing contaminated water from a well, volatilizing the contaminants in an air stripping tower, and adsorbing vapor-phase contaminants into granular-activated-carbon (GAC). There are limitations to this pump-and-treat method. The method is relatively inefficient, and some sites can require treatment for extended periods of time.
Chlorinated solvents can be degraded into less harmful materials by a method commonly referred to as "reductive dechlorination," in which chlorine is replaced by hydrogen. The reductive dechlorination uses metallic, solid reaction elements, such as iron and zinc, to degrade chlorinated solvents and other organic compounds. For example, Gillham, U.S. Pat. No. 5,266,213, discloses feeding contaminated groundwater through a trench containing iron to degrade contaminants. The Gillham process is conducted under strict exclusion of oxygen and occurs over a long time period. The Gillham process often requires large amounts of iron for complete reaction. Furthermore, it is difficult to introduce large volumes of solid reaction material, such as iron, using the Gillham process at effective depths for in situ remediation.
Another process proposed for removing contaminants from contaminated media is soil vapor extraction. In this process, contaminated media, such as contaminated soil is removed from the its location and treated to remove contaminants and vapor. The soil vapor extraction process is labor extensive and often is inefficient as recalcitrant fractions of the contaminants remain in the soil. Further, the soil vapor extraction process, as well as the above-described pump-and-treat process, is very difficult to use in some soils. For example, neither the soil vapor extraction process nor the pump-and-treat process, are particularly useful to treat contaminants in tight, clayey soils.
Several electrokinetic-based contaminated media treatment processes are known to have been used in attempts to treat contaminated media, such as treatment of chlorinated solvents, including hydrocarbons. An electrokinetic-based process typically moves charged particles through a continuous medium. A electrokinetic-based process also refers to a process in which charged particles are moved through a continuous medium over a charged surface. One electrokinetic-based process is the "Lasagna" process, which refers to a process that incorporates a plurality of treatment regions to destroy or adsorb contaminants between emplaced electrodes. The "Lasagna" process is disclosed in U.S. Pat. Nos. 5,398,756 to Brodsky et al. and 5,476,992 to Ho et al. The disclosures of these patents are incorporated herein by reference.
A "Lasagna" process typically includes the steps of (a) forming at least one liquid permeable region within a contaminated media, such as a contaminated soil region, (b) introducing material for treatment of contaminants in the contaminated media into a region of the contaminated media, such as a liquid permeable region, where the material typically forms at least one treating region within the contaminated media; and (c) applying and transmitting a direct electric current (DC electrical field) through the contaminated media between oppositely charged electrodes, wherein a first electrode is disposed at a first end of the contaminated media and a second electrode is disposed at the opposite end of the contaminated media. Thus, an electroosmotic flow is established from the second to the first electrode, and an in situ electrokinetic remediation of contaminated media, such as soil, is able to be achieved.
In situ electrokinetic remediation possesses several advantages with respect to other commonly applied remediation technologies. Contaminants can be removed from low permeability soils using in situ electrokinetic remediation. Also, fluid and ionic flow direction can be controlled using in situ electrokinetic remediation. Further, control of a flow direction is enhanced compared to common pump and treat systems because flows in in situ electrokinetic remediation follow electric field lines established by the application of the DC electrical field, which begin and end at electrodes. In contrast, flow in pump and treat systems is typically controlled and influenced by a pressure field in the contaminated media that can be adversely influenced by regions of high soil permeability and fissures through the contaminated media.
While electrokinetic remediation treatments possess desirable qualities with respect to pump and treat processes, their operation may be further improved by enhancing its flow direction control. For example, buried metal objects or regions of high-metallic content in the contaminated media may adversely distort the applied DC electrical field. This distortion may prevent uniform distribution of current in the soil. Also, electrokinetic remediation inherently heats the soil, and thus may create non-uniform contaminated media properties, including but not limited to, electrical conductivity and electroosmotic permeability. Non-uniform contaminated media properties may adversely influence the efficiency of the electrokinetic remediation process.
Therefore, elimination of the adverse influences of a remediation treatment process is needed.